1. Field of the Invention
The present invention relates to an electronic stethoscope capable of detecting and reproducing sounds of interest while eliminating most of the undesirable noise disturbing the physician or other medical practitioner during auscultation.
2. Brief Description of the Prior Art
A plurality of electronic stethoscopes have been proposed in the past. Examples are described and illustrated in the following United States patents:
______________________________________ 3,247,324 (Cefaly et al.) 04/19/1966 4,170,717 (Walshe) 10/09/1979 4,254,302 (Walshe) 03/03/1981 4,534,058 (Hower) 08/06/1985 4,594,731 (Lewkowicz) 06/10/1986 ______________________________________
Although many electronic stethoscopes are available on the market, they have never been widely accepted by the physicians and other medical practitioners. A clinical study in different fields of the medical profession indicates that non-acceptance of the electronic stethoscopes is mainly due to the production of noise (or artefacts) disturbing the physician or other medical practitioner during auscultation as well as to the incapacity of these stethoscopes to amplify and reproduce certain biological sounds of interest. These two considerations are not redundant. Indeed, a biological sound can be either present but covered by noise, or totally absent.
Many sources of noise have been identified. These noise sources usually have a high amplitude and/or a frequency characteristic situated within the frequency range of the signal of interest whereby the quality of auscultation is substantially reduced. Of course, an efficient electronic stethoscope should be capable of processing these different sources of noise.
"Noise" is defined as being any signal other than that of interest, and can be divided into the following four categories:
A. External noise; PA1 B. Noise related to auscultation; PA1 C. Noise generated by the electronic circuits of the stethoscope; and PA1 D. Noise of biological nature produced by the patient's body. PA1 1.degree. Noise related to the preauscultation manipulation; PA1 2.degree. Movements of the physician and/or patient (movements perceptible by human eyes); and PA1 3.degree. Tremor (involuntary trembling motion of the hand of the physician mostly imperceptible by human eyes).
A. External noise:
This external, ambient noise is not directly connected to auscultation, but originates from the environment of the physician, the patient and the stethoscope. For example, external noise is produced by the telephone, voice, medical equipments, etc. Its frequency characteristic is situated within the range 300-3000 Hz. These acoustic waves are sensed by the electroacoustic transducer (microphone) of the stethoscope and then amplified and transmitted to the ears of the physician or other medical practitioner.
Although it is attenuated by the stethoscope probe, the external noise is still perceived by the medical practitioner as being dominant since sensitivity of the human ear is higher at ambient noise frequencies. Reference can be made to the Fletcher-Munson curves showing that a sound at a frequency of 1 kHz is perceived up to 100 times louder than a sound of same intensity at a frequency of 100 Hz. Lower sensitivity of the human ear to the low frequencies of the biological sounds results in a weaker perception thereof.
The above mentioned clinical study has determined that, amongst the various types of noise, external noise is the most disturbing. For example, the sound of a normal heart has a frequency characteristic situated within the range of 20-200 Hz and is particularly covered and affected by ambient noise. Measure of blood pressure is particularly affected by external noise since Korotkoff's sounds have low frequency characteristics.
B. Noise related to auscultation:
The three following sources of noise are associated to auscultation of a patient:
1.degree. Noise related to the preausculation manipulation:
This category includes impacts between the probe (including the electroacoustic transducer) of the stethoscope and hard objects, and the noise produced upon adjusting the different controls (switches, potentiometers, etc.) of the stethoscope. In particular, when the probe hits an object the resulting sound is sensed by the electroacoustic transducer and is then amplified to produce a very high amplitude transitory signal. Under certain circumstances, the transitory signal can have an amplitude sufficient to harm the ears of the user. Moreover, when the controls (switches, potentiometers, etc.) are mounted close to the electroacoustic transducer, adjustment thereof is susceptible to produce very unpleasant artefacts if they are not adequately isolated from the electroacoustic transducer.
2.degree. Movements of the physician and/or patient (movements perceptible by human eyes):
Upon carrying out auscultation, noise can be generated when the physician or other medical practitioner positions and displaces the probe on the patient's body. Movement of the patient's body with respect to the probe produces the same type of noise. In both cases, the generated noise has a relatively high intensity. The power of these artefacts is surprising, in particular when the probe is positioned and then displaced on the clothes of a patient.
3.degree. Tremor:
Considerable efforts have been made to identify and characterize the source of a low frequency rumble wrongly associated to background noise. This low frequency rumble is generated by an involuntary trembling motion of the hand of the physician or other medical practitioner during auscultation, twinned with the sensitivity of the electroacoustic pressure transducer of the stethoscope.
When the stethoscope is applied to the patient's body by a physician or other medical practitioner, a low frequency rumble is produced and superposed to the biological sounds of interest. On the contrary, the low frequency rumble disappears when the probe of the stethoscope is held on the patient's body by means of an elastic belt instead of the physician's hand. This low frequency rumble is produced by an involuntary trembling motion (tremor) of the physician's hand, which trembling motion is mostly imperceptible by human eyes and has muscular origins (positioning feedback). It is interesting to note that the low frequency rumble considerably reduces in intensity when the probe of the stethoscope is held in the air; the explanation is that an electroacoustic transducer placed in a closed space is more sensitive to variations of pressure in the cavity than to the movement itself. Of course, when the probe of the stethoscope is applied to the patient's body, the involuntary trembling motion of the hand creates variations of pressure in the air compartment between the electroacoustic transducer and the patient's body. The electroacoustic transducer senses these pressure variations and generates in response thereto a low frequency signal of which the frequency characteristic is mainly situated in the range 10-100 Hz. When held in the air, the electroacoustic transducer is subjected almost only to static atmospheric pressure whereby the rumble considerably reduces and can even disappear.
C. The noise generated by the electronic circuits of the stethoscope:
This kind of noise includes harmonic distortion caused by saturation of the electronic circuits and modifying the signal of interest, as well as residual background noise of electronic nature superposed to the signal of interest. Proper design of the electronic circuits enables this noise to be, if not completely eliminated, considerably reduced.
D. The noise of biological nature generated by the patient's body:
As defined in the foregoing description, "noise" is any signal other than that of interest. Therefore, the sounds of biological nature produced by the patient's body can be considered as being noise. Accordingly when a cardiologist auscultates a low frequency sound (for example B3 or B4 heart sounds), muscular trembling, noise generated by the intestinal peristalsis, pulmonary sounds as well as high frequency heart sounds (murmurs, mechanical heart valves, etc.) all constitute noise which disturbs his concentration.
Generally, the prior art electronic stethoscopes have a frequency bandwidth covering the ranges of frequencies of the biological sounds of interest, mentioned in the following TABLE OF COMMON AUSCULTATORY SOUNDS. This well-known table identifies the frequency contents of the cardiac, respiratory and fetal sounds of interest. To those sounds should be added the Korotkoff's arterial sounds auscultated upon measuring blood pressure and mainly situated in the frequency range 20-150 Hz. Accordingly, the prior art stethoscopes designed from the information given in this table present various frequency responses comprised between 20-2000 Hz. ##STR1##